Changes in intracellular calcium were measured in voltage clamped giant molluscan neurons with the dye arzenazo III. Changes in potassium currents were also measured and correlated with internal calcium concentration increases for membrane potentials up to about +30 mV and thereafter decreases with a suppression potential of about +140 mV in normal external saline. The average changes in internal calcium at +30 mV was about 330 nM and we estimate that at the inner membrane this value is about 21 times greater at the end of a 300 ms ec depolarization. Internal calcium activates a change in potassium conductance which is voltage dependent. Our results indicate that the calcium activated potassum conductance increases at positive membrane potentials. There is an efold increase in this conductance per 25 mV change in potential. Other ions besides calcium can activate this conductance, but less effectively than calcium. We find that Cd2 ion, Hg2 ion, Sr2 ion and Mn2 ioN activate whereas Mg2 ion, Co2 ion Zn2 ion, Cu2 ion and Ba2 ion are ineffective. The sequence is Ca2 ion greater than Cd2 ion greater than Hg2 ion greater than Mn2 ion greater than Fe2 ion and suggests that the binding site for activation of the potassium conductance can accommodate ionic radii between 0.76 A degrees to 1.13 A degrees.